Acoustical firing indicator



Feb. 16, 1960 Filed Feb. 18, 1957 J. I. MATTE] ETAL ACOUSTICAL FIRINGINDICATOR FIG. 1

4 Sheets-Sheet 1 IITTOF/VEY Feb. 16, 1960 J. I. MATTEl ETAL ACOUSTICALFIRING INDICATOR Filed m. 18, 1957 4 Sheets-Sheet 2 Q 9 5 3 ,2, E 6''2', A,

3 ll 3 I 11 12 FIG.7 FIG.8

FIGA F165 INVENTOR JEANLMATTE/ JEAN BLA/S'E RENE MA/VGA NNE ATTORNEYFeb. 16, 1960 J. l. MATTEI ETAL 2,925,582

ACOUSTICAL FIRING INDICATOR Filed Feb. 18, 1957 4 Sheets-Sheet 3 FIGJOF16-" JEA/VZ MATI'E/ c/E/M/ 5L 4 I35 PEA/E MANG/M/NE ATTORNEY Feb. 16,1960 J- l. MATTEI ETAL 2,925,582

ACOUSTICAL FIRING INDICATOR Filed Feb. 13, 1957 4 Sheets-Sheet 4 FIG-12pa 2Q //VVf/V7'0?$ dEA/VI. M47767 JAN BLA/SE yf/VE MANGA/VA/E ByArroE/VEY United States Patent O ACOUSTICAL FIRING INDICATOR Jean I.Mattei, Vincennes, Jean Blaise, Paris, and Ren Manganne,Chatillon-sous-Bagneux, France, assignors to Ofiice National dEtudes etde Recherches Aeronautiques, Chatillon-sous-Bagneux, France, a FrenchThe present invention deals with an'air-air or airground firingindicator.

When a firing exercise has been carried out on a real target, thebenefit derived therefrom is increased according to the amount ofinformation gained about the respective positions of the target and ofthe projectiles that have been launched toward it. It is desirable toknow the paths ofprojectiles that did not hit the target and whichaccordingly did not leave any trace on it.

The object of the present invention is a device which registers thetarget area, either in an instantaneous and approximate manner visibleto the firing operator, or in a more exact fashion on a recordingapparatus which may be inspected after firing, or in both ways.

This invention is applied to firing with supersonic projectiles.

It is known that an object moving in the atmosphere at a supersonicspeed creates a disturbance in the form of a ballistic shock wave. Thewave, in general, has the following structure:'

The atmosphere rises sharply from a static pressure P to a higherpressure, Pd-I-p. Then the pressure decreases linearly, both in time andspace, to P5-p 'and returhs sharply to P This structurecommonlyjiscalleda shock N-wave in the case of small projectiles;the complex' disturbancemay involve three or four pressure jumps. The amplitude of the pressurejump and the interval, separating the straight edges depend on thecharacteristics of the object, its caliber and speed, and on the normaldistance of the trajectory from the point of detection. V

Miss distance indicators based on the amplitude variation of the N wavealready are known. A device of such type is described, for instance inan article of Markus 'C. Eliaphon et a1. entitled Acoustic Firing ErrorIndicator, in Electronics, vol. 25, No. 10, pages 98-101, October 1952.

The present invention does not utilize the variation in amplitude formeasurement, but the variation in time duration of the N wave as afunction of distance. Measurements of time duration are less subject toerror in collection and transmission of results than measurements ofamplitude. On the other hand, the former require great precision incollection and transmission of the time-data of the two rigid edges ofthe N wave. For this reason, the microphones picking up the passage ofthe N wave have a very broad band; and the transmission of data. iseffected by frequencymodulation.

The microphones are placed about the target. Four in number are shownand they are placed on a circular crown. .Such an arrangement makespossible a close-up presentation of the target area which is suitablefor immediate. use by the firing personnel and later for the recordingin a more exact manner which, however, is not immediately available tothe firing personnel.

The immediateindication is made in the form of a parametric planerepresentation of the target area. This parametric representation" isconceived to' permit' an im- 4 2,925,582 Patented Feb. 16, 1960 icemediate summary interpretation by the firing operator. It may beaccomplished by simple means, permitting installation aboard a fighterplane.

Theinvention will be better understood in connection with the detaileddescription which follows and with the annexed drawings, in which:

Fig. 1 is a graphic representation of a ballistic N wave; Fig. 2represents a perspective view of an array of four microphones;

Fig. 3 represents the electrical equivalent circuit of the microphone;

Fig. 4 represents a cross-section of the assembly of monoammoniacphosphate plates in a microphone;

Fig. 5 is a cross-sectional view of one of the electrodes;

Fig. 6 is a cross-sectional view of one of the micro phones;

Fig. 7 is aperspective view of one of the microphones;

Fig. 8 is a cross-sectional view of a microphone;

Fig. 9 is a schematic diagram of the firing indicator;

Fig. 10 represents the signals as produced in the receivers afterdetection;

Fig. 11 is an example or" a firing indication;

Fig. 12 is an example of a transparent graduated mask, permitting theinterpretation and use of the immediate parametric result furnished forthe use of the firing operator.

Fig. 1 is a representation of a ballistic shock wave produced by aprojectile traveling at supersonic speed.

The abscissa line .is plotted either in units of time or in units ofdistance. If it is in units of time, T represents the duration of the Nwave. 27 represents an N wave having two straight edges, 29 and 30.Further details on N waves are given for instance in an article entitledA Determination of the Wave Forms and Laws of Propagation andDissipation of Ballistic Shock Waves, in the Journal of the AcousticalSociety or America, vol. 18, -No. 1, July 1946.

. In Fig. l, 28 represents the signal reproduced by.the

microphone withlessthan 10% distortion.

'-.In Fig. 2, a crown 45 is shown on which are mounted four microphones41', 42, 43, and 44, placed at the extremitiesof two diameters at rightangles. Each of these microphones has an identical adaptor, shown as 51,for microphone 41. The other identical adaptors are omitted for reasonsof simplicity. The terminals of the adaptors are connected each to itscorresponding transmitter; a transmitter is represented schematically as61.

Microphones should be selected of a type capable of reproducing aballistic N signal having a duration of between 0.1 and a fewmilliseconds, with a distortion of less than 10%.

The following approximate characteristics are necessary: Sensitivity: 20rnicrovolts per bar; and a pass band from 20 c./s. to kc./s.

These characteristics may be obtained with microphones containing anarray of approximately cubic shapes of small flat square plates ofmonoammoniac phosphate. It is known that monoammoniac phosphate, oftendesignated by the abbreviation A.D .P. for ammonium dihydrogenphosphate, is used in the form of a piezoelectric crystal. Reference ismade particularly to "Piezoelectric Crystals and Their Application toUltrasonics by Warren P. Mason, D. Van Nostrand Co., Inc., 1950, pages137 to 164.

By adopting the approximately cubic form, the multiple oscillations dueto resonance between the various modes of mechanical oscillations whichmay be set up in the crystal are eliminated.

he plates may be shaped according to cross-section Z 45, as shown onpage of the above cited reference, in such a way that, once theballistic shock pressure is applied to the field of the plates, thepiezoelectric current will be collected on the large square faces,against which the electrodes are arranged.

In Figs. 6 to 8, 1 designates the block of crystals of monoammoniacphosphate, and 2 achamber of insulating material 'on the botto rnofwhich the block is mounted with a--'flexible adhesive. 'It is furtherkept in place by supple spacers 3. made for instance'of cork. i

Thecrystals have the form of parallelepipedic plates; there aresix'mounted in the model described and their assembly forms a cube,which, in the model of the invention, is 1 m Asa result, the thicknessof the plates is approximately 1.6 mm. The crystals'are shaped, asabove, as per cross-section 2 45. Block 1 is positioned in the chamberin such a way that the ballistic shock wave falls on the edge of theplates.

The housing 2 is inserted into a cylindrical metallic mount '4, which isscrewed to a coaxial contact socket 5.

Oneof the connecting wires 7 of the microphone passes I face of thespacers and the active face of the crystals.

Plates 13 to 18, shown in Fig. 4, are arranged in parallel andelectrodes 19 to 25, made for instance of silver leaf, are placed on theoutside faces of plates 13 and-'18 and interposed between the adjacentfaces of the difierent plates. Two adjacent plates are mounted in suchaway that, while receiving pressure on their fields side by side, thepiezoelectric charges gathered on the large adjacent faces, separated byan electrode, are of the same polarity.

QEach electrode in Fig. 5 comprises a snap contact, 26 'for'electro'des19, 21, '23, 25, and 26' for electrodes 20, 22, 24. Wire 7 is solderedto contact 26, and wire 8 tocontact 26' as shown in Fig. 4.

Referring to Fig. 3, an electric-circuit, suitable'for a completecrystal, comprises a signal source 35, an

inductancerL a resistance R and a condenser. C in series, a shuntcondenser C and a leak resistance R The phantom C represents the straycapacity.

. If we assume that block 1 is formed of a single cubic crystal equippedwith the-two electrodes '19 and 25,'and

if we designate the charge appearing on the electrodes for a givenpressure as q the voltage generated is:

where C designates the capacity of the condenser constitutedbyelectrodes 19 and 25 separated by the block of r'nonoammoniac phosphate.For the'cubic block constituted by n plates in shunt, the capacity Cbecomes n C since the capacity of a plate is nC Moreover, the f'chargebecomes nq Subsequently the voltage generated becomes:

i nCo+C If we assume we get:

v w-ill be maximum for:

apaassa which gives:

in the case of the block with n plates. The product:

has not varied in relation to its value in the case of the single block.It has to be of the order of 10 times the duration T of the longestsignal in N, or 75 milliseconds. C being, as we have seen, of the orderof 1 pf. for a cube ofl cm. and the resistivity of monoammoniacphosphate, equal to the resistance ofa block 1 cm beingof the order of4.10 we obtain 40 milliseconds, which is easily of the order of the sizedesired for the product C R As has been indicated in relation to Fig- 2,each microphone is connected with an adaptor which permits passing theoutput from the high impedance of the microphone to the impedance of thefollowing transmission circuit. It is, for example, a cathode-followerstage of conventional type. It also serves as support forthernicrophone. The ensemble is mounted on rubber to limit vibrationsand transmission of sound through the inner surfaces. 7

An adapter 51 is linked with a corresponding transmitter 61.- Thetransmitters are frequency modulated. .A suitable characteristic is:excursion of frequency, 300 kc ./s., and output 2 watts, for example,which permits apractic al range of 30 km.

The receiver will now be desccribed in relation to Fig. 9. There arefour receivers 71 to 74, each with frequency modulation. Afterdemodulation, each furnishes a signal similar to the abscissa line 28 ofFig. l. The output signal is differentiated in circuits shownschematically as 81 to 84, having pulse shapes represented as 3 1 to 34of Fig. 10. Each of these pulses has two short pips hereinafter definedas leading pip and trailing pip, respectively. The first leading pip isshown to origi nate from the microphone closest to the trajectory, inthe present case microphone 41. Taking the arrival of the first leadingpip as the beginning time of the sequence of arrival of the pulses, theleading pips of the remaining channels are retarded at t t and trespectively. The time interval between the leading and trailing edgepips of a same signal increases with the distance of the microphone fromthe origin of this signal.

A time base 101 corresponds to receiver 71. If a gate circuit 91 is openthe time base may be triggered. At the 'rnoment when time base 101 istriggered, it closes gate circuit 93 of the time base 103, opposing it.The link is shown schematically as connection 75.

Similarly, there is a time base 103 corresponding to receiver 73. Thisreceiver can trigger its time base when gate circuit 93is open. As soonas the time base is released'it closes the circuit of gate 91 throughcircuit connection 76, associated with time base 101. The time bases 102 and 104, and the gate circuits 92 and 94 operate in a manner similarto the above.

ifll .to 104 will use signals of positive polarity and act onthedeflection plates 1 11 to 114 of cathode-ray tube 110. 3 t

Assuming that for thepassage of a certain projectile the signals issuedfrom computers 81 to 884 are those represented in 31 to 34 in Fig. 10,it is the time base 101, which is triggered first, then after time 1time base 102. At period 1 time base 103 cannot be triggered, since whentime base101 is triggered, this will close gate circuit 93. Similarly,time base 104 will not have been triggered since the gate circuit 94 isclosed.

The time bases are conceived so as to be stopped and returned to theiroriginal situation by the trailing edge pip. Thus, when the signals ofFig. have run their course, condition of the circuitry of the indicatorreturns to its initial state, ready to function on the next shock wave.

Fig. 11 represents the indication given by the indicator which will havereceived a shock wave corresponding to Fig. 10. The spot has a tracedcurve 120 comprising a horizontal section representing the period fromzero to t and an oblique section representing the period from I; to theinstant of occurrence of the trailing edge pip of the N wave received bymicrophone 51. As this is the microphone reached first by the wave, thetrajectory has accordingly passed to the left of the target and asmicrophone 42 was reached before microphone 44, the trajectoryaccordingly passed over the target. Microphone 41 has been reachedbefore microphone 42; the assumed point of impact was below the planemidway between these two microphones. This appears on the screen shownin Fig. 11 placed under the eyes of the firing operator.

In order that the firing operator may evaluate the target-trajectorydistance, a grid of the type shown in Fig. 12 may be conveniently placedupon the screen of the cathode-ray tube. The equidistant lines areclosed. Their representation depends on the law of variation of thequantity T with the distance.

On the graphic mask of Fig. 12 is represented a traced designcorresponding to mm. shells. The equidistant curves are squares, and thedistances corresponding to them are indicated by numbers, in meters, forexample. It is possible to color the different zones diiferently. A spottrack is represented in 120, similarly to that of Fig. 11.

Working from this distorted parametric representation, it is possible toreconstruct the exact target area. This requires an installation whichis somewhat bulky for a fighter plane.

The method of operation may occur in the following manner: Parallel tothe immediate indicator of fire, there is an indicator the screen ofwhich is registered photographically. According to circumstances, thesecond indicator may use either the same receivers as the first orseparate receivers. 'The target area is reconstructed by a translatoraccording to the results of the photographic records.

As a variation, it is likewise possible to register the signals of Fig.10 directly. A fifth track furnishes the time base. These elements arefurnished as data to a translator-calculator.

It should be understood, of course, that the foregoing disclosurerelates to only a preferred embodiment of the invention and that it isintended to cover all changes and modifications of the example of theinvention herein chosen for the purposes of the disclosure, which do notconstitute departures from the spirit and scope of the invention setforth in the appended claims.

the said target in a plane at an angle to the trajectory of said bullet,each said transducer including electric circuitry means to separatelydetect the passing of said leading edges and of said trailing edges of asaid N shock wave and the time-lag between them, source of energy meansto produce electrical outputs in response to the detected said leadingand trailing edges of said wave, and proportionally in time to thetime-lag between said two edges, receiver means comprising separatemeans to receive the said electrical outputs of each said transducer,computer means for each one said transducer to calculate the duration ofeach said N shock wave from the time elapsed between its leading andtrailing edges, means to compare the duration of each said N shock wave,means to compute the target miss distance and duration of travel of eachsaid passing bullet, when closest to the said target, on the basis ofthe law of variation of the duration of said N shock wave as a functionof distance.

2. A target miss distance and direction indicating system as defined inclaim 1, further comprising a transmitter for each said transducer,electrical circuit means connecting each said transducer with each saidtransmitter.

3. A target miss distance and direction indicating system as defined inclaim 2, further comprising a receiver means, one for each saidtransmitter.

4. A target miss distance and direction indicating system as defined inclaim 1, comprising recording means for subsequent analysis of targetscoring from the outputs of said transducers and electrical circuitmeans connecting said transducers with said recording means.

5. A target miss distance and direction indicating system as defined inclaim 4, where the said each transmitter and said each receiver isfrequency modulated.

6. An acoustical firing error indicator for indicating the distance anddirection of a supersonic bullet relatively to a given artificial targetcomprising four microphones assembled on a circular crown secured tosaid target, four frequency modulation transmitters operating onseparated radiofrequency carriers fed by said microphones, a re ceiverset including four frequency modulation receivers, each of them tuned atone of said transmitter radiofrequency carriers, four difierentiatingcircuit means each deriving a first pulse signal from the said leadingedge of the N-wave signal, and a second pulse signal from the saidtrailing edge of said N-wave signal, four time base circuit means eachtriggered on by said first pulse signal and triggered ofl by said secondpulse signal, electrical circuitry means to group said four time basemeans into two pairs with only one of each of the said pairs of saidtime bases capable of triggering on at once, a cathode ray tubeindicator with two sets of deflecting plates, means to apply time basesignals issuing from said time base means of the first said pair to thefirst set of deflecting plates and time base signals of the second saidpair to the second group of deflecting plates.

7. A target score system comprising apparatus means as defined in claim6 and further comprising associated means to video plot said targetrelative to said miss distance and direction of said bullet.

8. A system for detection of target miss distance and direction intarget practice of firing bullets at supersonic speeds creatingsupersonic N shock waves having a leading and a trailing edge, saidsystem comprising an array of four piezo electric crystal microphonesspaced in a prefixed relationship to each other and relative to the saidtarget in a plane at an angle to the trajectory of said bullet, fourelectric circuitries, one for each said microphone, to produceelectrical outputs responsive to said leading and trailing edges,including transmitting means of said outputs, receiver means comprisingseparate means to receive said electrical outputs of each saidmicrophone, computer means for each one said microphone to calculate theduration of each said N shock wave from the time elapsed between itsleading and trailing edges, means to compare the durations of each saidN shock 7 wave, means to compute the target miss distance of each saidpassing bullet, when closest to the said target, onthe basis of thelawof variation oft-he duration of-sa'id N shock wave as a function ofdistance.

9. A target miss distance and direction indicating system as defined inclaim 8, further comprisinga transmitter for each said transducer,electrical circuit I means connecting each said transducervwith eachsaid transmitter.

10. v A target miss distance and direction indicating systemaas definedin claim 8, comprising recording means for subsequent analysis of targetscoring from the outputs of said stransducersand electricalicircuitmeans connecting said transducers with said recording means.

References 'Cited in the file of this patent I Journal ofthe AcousticalSociety of America, vol. 18, No; 1; July 1946, pp. 97118.

